Mark non-captured constexpr variable as static

Summary:
GCC & Clang allow the use of a non-captured constexpr local within a lambda, MSVC does not.

All 3 compilers allow this when the local has static storage.

Reviewed By: yfeldblum

Differential Revision: D4675147

fbshipit-source-id: 3e07eb77731cbd3ebda6aee4b0b57ac19ac34de5

folly/experimental/test/EliasFanoCodingTest.cpp

@@ -46,7 +46,7 @@ uint8_t slowDefaultNumLowerBits(size_t upperBound, size_t size) {
 TEST(EliasFanoCoding, defaultNumLowerBits) {
   // Verify that slowDefaultNumLowerBits and optimized
   // Encoder::defaultNumLowerBits agree.
-  constexpr size_t kNumIterations = 2500;
+  static constexpr size_t kNumIterations = 2500;
   auto compare = [](size_t upperBound, size_t size) {
     using Encoder = EliasFanoEncoderV2<size_t>;
     EXPECT_EQ(int(slowDefaultNumLowerBits(upperBound, size)),